The present invention relates to a free-flow pump, wherein a free flow chamber permitting free passage between suction and pressure pipes is located in the housing of the pump laterally of the impeller which is disposed in an impeller chamber, the greatest diameter of the free flow chamber exceeding the diameter of the impeller chamber. Known pumps of this type (US Pat. No. 3,171,357) generally exhibit a disadvantageous throttle curve, particularly when they have a relatively large, free passage, i.e. when the following condition is met: ##EQU1##
The disadvantageous throttle curve is caused by malfunctions during partial load in the region of the housing tongue, and such malfunctions affect the impeller flow. The use of smaller impeller outlet angles may in fact stabilize the throttle curve to some extent, but losses in head and in efficiency cannot be prevented owing to the reduced vane loading associated with smaller impeller outlet angles.